Rigid Body and Invisible Object Collision
As of now, I see no reason why you couldn't add a Transparent BSDF shader to the smashy object.

View:



Render:



Animation with transparent smashy thing:



My .blend file:



Also, as lemon points out, in the Properties panel > Object tab > Cycles Settings dropdown, you could uncheck the Camera checkbox. If you follow this course, though, you will have a disembodied shadow.



As Nathan Craddock adds, you can also turn off Render Visibility in the Outliner.